The present invention generally relates to electrical resistors, and particularly relates to power resistor applications.
Power resistors find broad application across a variety of system types and applications. In many electric motor applications, power resistors are used as braking loads for the motors. In other applications, such as in frequency converters, power resistors are used in snubber circuits that protect high-power switching devices by suppressing voltage and current spikes arising from line switching actions.
While these and other applications differ significantly in terms of end purpose, many of the design and operating challenges imposed on power resistors are common across the range of uses. Power resistors must generally provide reliable, safe, long. term operation under rated power conditions, which often entails extended operation at high power levels. Because the dissipation of electrical energy through resistance involves converting electrical energy to thermal energy, power resistor reliability depends on good thermal management.
Often, power resistors are required to dissipate so much electrical power that the resultant heat would be damaging absent some form of heat sinking. Heat sinking entails placing a heat-generating object in thermal contact with a heat-dissipating object. In practical terms, for power resistors, this often entails placing the power resistor in good thermal contact with a larger heat radiator, or even in contact with a liquid cooled heat sink.
In whatever configuration successful thermal management depends on the efficient conduction of heat away from the power resistor and into the heat sink. Without good thermal conduction, operating temperature of the power resistor may rise to dangerous and damaging levels, which is unacceptable in any practical system.
Many techniques exist for enhancing the thermal performance of power resistors. First, the resistors themselves may be made to have good intrinsic heat dissipation characteristics. Imbuing such characteristics conventionally entails making the resistor larger, which has obvious disadvantages in size-constrained systems. This is exacerbated by the tendency to use banks of power resistors, rather than just one or two such devices in a given system.
Other approaches focus on establishing a thermal conduction path between each power resistor and the heat sink. Maintaining good heat flow requires minimizing thermal resistance at the junction between the power resistor and the heat sink. Techniques for accomplishing this minimization include the use of thermal bonding compounds, along with mating the power resistor to the heat sink under relatively high contact pressure. Inherent in these approaches is the notion of providing generally smooth, flat mating surfaces between resistor and heat sink.
Arguably, too many tradeoffs arise from the above considerations. Installation considerations limit the size and therefore intrinsic heat dissipation capability of power resistors, which imposes the requirement to efficiently conduct heat out of the power resistor into a heat sink. Thus, power resistor package must comprise materials that provide good thermal conduction, yet the need to minimize thermal contact resistance requires relatively high contact pressure requirements. The resultant mechanical stresses suggest the need for mechanically robust power resistor packages, but this must be balanced against the thermal properties of the materials used.
Thus, a power resistor that embodies good thermal conduction, mechanical robustness, and small size is needed. Preferably, this power resistor would be relatively simple to manufacture, and would accommodate various mounting arrangements. The present invention addresses these and other needs, as will be made evident later herein.
A power resistor includes features that enhance its performance in high-power electrical systems, and may be formed in a stacked arrangement with opposing terminal plates. Generally, the power resistor includes two terminals for contacting with an external system, and a resistor element providing the desired electrical resistance between the two terminals. Preferably, an electrical insulator is positioned between each terminal and the resistor element to prevent electrical shorting between the two terminals across the resistor element. By using electrical insulators with favorable thermal conduction characteristics, the insulators provide efficient thermal conduction paths from the resistor element into the two terminals, one or both of which may be in contact with an external heat sink.
In some embodiments, the insulators are made from aluminum or other metal to capitalize on the good thermal conduction and mechanical strength of metal. A surface treatment, such as an anodization process for aluminum, is used to render the metallic insulator""s surface nonconductive. The use of treated metal as electrical insulation within a power resistor structure provides the power resistor the ability to withstand compressive mounting forces without need for special precautions, as well as providing good thermal conduction between the resistor element and the terminals.
When implemented in a stacked arrangement, the component pieces comprising the power resistor are preferably joined by mechanically pressing them together. An exemplary stack includes top and bottom terminals with a resistor element positioned between them, and with an insulator positioned between each terminal and the resistor element. The different elements within the stack include mechanical features that establish the desired electrical contact points between the two terminals and the resistor element, and that further provide the inter-element contacts that join the stack when mechanically pressed together.
In an exemplary stack arrangement, the terminals, insulators, and the resistor are all substantially flat plates or discs that stack together. In this arrangement, the top or first terminal has a raised projection on its inner surface that is preferably centrally located. The insulator disposed between this first terminal and the resistor element has a central cutout or opening that exposes the resistor element. The terminal""s projection passes through the opening, making mechanical and electrical contact with the resistor element. The resistor element may include a opening corresponding to the shape of the terminal""s projection and sized to allow the resistor element to be pressed onto the projection, thus fixing the resistor element to the top terminal, with the intervening insulator sandwiched between them.
Similarly, the bottom or second terminal has a perimeter lip formed on its inner surface, with the lip defining an inset area or recess. The second insulator is sized to fit into this recess and is positioned therein. In turn, the resistor element is sized such that its outer perimeter conforms to the terminal""s perimeter lip, and just matches or is slightly larger in size than the inset area. Thus, the resistor element may be joined to the second terminal by pressing it into the inset area. In this manner, only the outer perimeter of resistor element is in electrical contact with the second terminal.
When implemented in the above stacking arrangement, the power resistor comprises a small, mechanically robust package that is well suited for high power applications. The power resistor is well suited for continuous power dissipation, and for operation subject to high power level transient voltage spikes. Its use of thermally efficient electrical insulators to draw heat from the resistor element into the two terminals allows the power resistor to dissipate very high levels of electrical power, provided proper heat sinking measures are taken. The preferably flat and outwardly smooth terminals complement heat-sinking arrangements by providing relatively large, low thermal-resistance surface areas for contacting the power resistor.